Uneven floor problems!

I have a problem with my dinning room floor. I want to Put in new hard wood floor, but it seems the old owners had an issue and repaired the floor the best they could. It's uneven in a section about 2 ft by 2 ft! What do I do to fix this before new flooring is installed?

It would be better if you go for refinishing the floor, and if you want to add the picture
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There are many options on the bar click on the attachment icon some option will open
Browse the image and upload, Picture will appear like this.
Hopefully you get the idea. I have uploaded pic for your understanding